1. Field of the Invention
The invention relates to the online detection and measurement of three dimensional (3D) spatial points of an object surface in a 3D reference system.
2. Description of the Related Art
In a conventional photogrammetric offline system, the object to be measured is recorded by a user, who holds the camera freely, from various directions. In this case, the measurement points on the surface have to be signalled physically beforehand. In a later evaluation step, the 3D coordinates are then calculated for these signalled measurement points.
The photogrammetric offline system has the disadvantage that the measurement points have to be signalled physically. This requires a great deal of time. A further disadvantage is that the 3D coordinates of the measurement points are not available until significantly later in time than the actual recording of the object to be measured. This is because the recorded images cannot be subjected to further processing in the computer until later.
In order to determine coordinates in only one plane or in only one direction, German patent document DE 195 14 815 A1 discloses a coordinate measuring device having a measuring head, which can be moved on a guide unit along a scale on a slide, and having a probe for measuring a workpiece. The probe is fastened to a swivel body which is arranged such that it can swivel about a pivot on a slide which can be moved on the guide unit. Arranged on the swivel body are measuring heads assigned to the scale on both sides of the pivot, the measuring heads being arranged at a distance from the scale and from the pivot, in such a way that during measurement, the swivel body is displaced parallel to the scale, one coordinate being detected as a dimension in the longitudinal direction of the scale and evaluated for the purpose of coarse positioning. Once the probe has been placed onto the surface of the workpiece, the swivel body is swivelled about its pivot. For the purpose of fine positioning, one coordinate at right angles to the surface of the scale is detected as a measure of the distance between the measuring heads and the scale and evaluated. The measuring heads are designed as measuring cameras in which defined markings of the scale are projected, with the aid of an optical system, onto an image plane formed by a CCD linear array. The position and the height of the projection center of the measuring heads are determined from the measurement signals with the aid of a computer.
A conventional photogrammetric online system is described in, for example, U.S. Pat. No. 5,440,392, in which it has usually been necessary for arranging at least two cameras on a tripod or a similar device to be installed permanently, with installed cameras observing the object to be measured. The measurement points on the object to be measured are touched by a user with a mechanical probe on which there are measuring marks. The coordinates of these measuring marks are automatically determined and, therefore, so too is the position of the measuring tip and the coordinates of the measurement point.
A significant disadvantage of this online system is that the cameras have to be constructed such that they are stable and stationary at all times, and that the cameras permit only a limited volume to be measured, specifically only the area which is covered at least by two cameras. At present, this area is only approximately 1 m.sup.2 in size if accuracies in the region of less than 1/10 mm are to be achieved. Since, in practice, this volume to be measured is frequently insufficient, the position of the cameras must be changed in order to be able to detect a wider area in the volume to be measured, for example, in the case of an object to be measured formed by a vehicle. However, this moving of the cameras requires that the system be completely recalibrated each time, and is thus particularly time-consuming and inconvenient.